As is known in the art, air traffic control is a service to promote the safe, orderly, and expeditious flow of air traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground; assisting aircraft in avoiding hazardous weather; assuring that aircraft do not operate in airspace where operations are prohibited; and assisting aircraft in distress. Orderly and expeditious air traffic flow assures the efficiency of aircraft operations along selected routes. It is provided through the equitable allocation of resources to individual flights, generally on a first-come-first-served basis.
As is also known, air traffic control services are provided by air traffic control systems. Air traffic control systems employ a type of computer and display system that processes data received from air surveillance radar systems for the detection and tracking of aircraft. Air traffic control systems are used for both civilian and military applications to determine the identity, location, heading, speed and altitude of aircraft in a particular geographic area. Such detection and tracking is necessary to direct aircraft flying in proximity of one another and to warn aircraft that appear to be on a collision course. When the aircraft are spaced by less than a so-called minimum separation standard (MSS) the aircraft are said to “violate” or be in “conflict” with the MSS. The MSS separation can be measured in distance or time, but MSS is typically a time separation standard within the terminal area. In this case the air traffic control system provides a so-called “conflict alert.”
Conventional systems such as the En Route Automation Modernization (ERAM) system in the United States and the Canadian Automated Air Traffic System (CAATS) in Canada provide control of IFR (instrument flight rules) aircraft outside terminal air space. These conventional systems additionally schedule and sequence the entry of these aircraft into the terminal airspace, which generally extends 20-40 nautical miles from an airport. Conventional procedures provide separation outside the terminal airspace and provide spatial displays of the relative locations of aircraft within a selected field of view. The separation requirements inside terminal air space are different from separation requirements used outside terminal air space due to lower aircraft speeds, dense air traffic, and shortened time intervals between aircraft. Within the terminal airspace, air traffic controllers manage the separation of aircraft using situation displays that receive processed data from surveillance radars and other air traffic control (ATC) systems.
Examples of managing the separation of aircraft within the terminal air space include managing situations where respective flights are individually assigned to and are following each of two published approaches that lead to crossing runways. In the crossing runway example, the controller must space arriving aircraft so that two aircraft do not arrive at the point of intersection at, or nearly at, the same time. Other examples include situations where two streams of traffic are approaching a pair of closely spaced parallel runways or where two or more streams of traffic are converging on a final approach course. In the United States, and other countries, flights arriving at busy airports are typically assigned scheduled arrival times before entering terminal airspace. This establishes an arrival sequence at the airport and permits the terminal controller to focus on maintaining required separation between consecutive arriving aircraft. In the situations described above, the air traffic controller can direct an aircraft to alter its speed, heading, or to switch to another approach in order to maintain a required separation time interval. By properly spacing aircraft, the air traffic controller can maximize the use of resources within the terminal air space while maintaining safety.
An air traffic control tool, called the Traffic Management Advisor (TMA) developed for the Federal Aviation Administration (FAA), provides a time-based display. TMA is intended as an aid in sequencing and scheduling flights that are up to 200 nautical miles distant from their destination airport. For a given flight, the TMA display indicates both the estimated time of arrival (ETOA) for that flight from its current position to some reference point and a corresponding scheduled time of arrival (STOA). The controller then attempts to reduce the difference between the ETOA and the STOA by giving speed change or course adjustment directives to the pilot of the aircraft. TMA is a scheduling and sequencing tool for aircraft before they enter the terminal area for their destination airport and is not used for controlling aircraft within terminal air space. TMA does not provide any indication of required separation, forward or aft, for an aircraft.
U.S. Pat. No. 4,890,232 describes spatial displays to aid air traffic controllers by projecting “ghost” images of flights that are arriving on a first approach, onto a representation of a second approach carrying actual arriving flight traffic converging on a common reference point. The air traffic controller can then provide separation between ghosts and real flights. If the ghosts are correctly projected, this will ensure that no conflicts occur at points where the approaches converge. This approach does not use time-based displays nor does it provide any direct indication of the transit time for flights.
U.S. Pat. No. 4,890,232 describes the use of situation displays that are normally used by air traffic controllers and projection of additional (ghost) images onto the display. However, since these tools are using distance-based displays, there is a problem regarding the placement of the images or ghosts. The problem is caused by variations in the ground speeds of flights in a terminal area. For example if a ghost of a slow moving flight is projected onto an approach where fast moving flights are operating, it is readily not apparent as to whether the ghost should move at the speed of its parent flight or at the speed of aircraft that are on the same approach as the ghost. If the ghost moves at the same speed as its parent flight, a fast moving flight may overtake the ghost before it reaches its reference point. This may cause the controller to divert the faster flight even though there is no real conflict. On the other hand, if the ghost of the slow moving flight travels at a speed that depends on the traffic on its approach then it is not apparent how the speed of the ghost should be calculated or where on the display it is to be placed. There may be more than one flight on that approach and the speeds of these flights may differ. Moreover, the flight speeds generally vary with time. Consequently, there are significant disadvantages to placing ghost images on distance-based displays to serve as a traffic separation tool.
Another problem with spatial or distance-based displays is that it may be difficult to estimate the time separation between flights. This is because flights typically decelerate as they approach the respective intended runway. In fact, the flight speeds decrease by fifty percent or more while they are in the terminal area and before landing. This is the cause of the phenomenon called “traffic compression” where the distance between consecutive flights decreases, as does their distance to the runway. With a time-based display, the displayed “distance” between flights will remain constant, on the average, as they move towards the respective destination runway. Therefore, in certain air traffic control applications, time is the preferable parameter by which aircraft separation should be measured and displayed.
A number of automated air traffic control systems, including the Standard Terminal Automation Replacement System (STARS) of the United States FAA, are capable of automatically alerting controllers of potential conflicts between two flights. A conflict arises when there is insufficient altitude and distance separation between flights. Such tools are primarily intended to warn controllers of situations where the intended paths of two flights cross at the same point and at (nearly) the same time. These collision avoidance tools are not intended to be used as an aid in separating two or more streams of converging air traffic. For terminal operations, collision avoidance tools are likely to either generate too many alerts or too few alerts depending on how they are configured and applied to a particular terminal configuration.
It would, therefore, be desirable to provide a time domain display aid to assist air traffic controllers in spacing two or more streams of aircraft that converge, cross or otherwise come within close proximity within terminal airspace. It would be further desirable to display an indication that there is insufficient time separation between respective flights that are nominally following or destined to follow the same approach or closely spaced approaches, to provide indications of the likely errors of the estimated transit times for each flight between its current position and a reference point, and to use the error information to improve spacing of the aircraft.